When a chimpanzee goes to sleep, it first has to build a "nest", which allows it to sleep safely up in the trees. Strangely, chimpanzees also build nests when sleeping on the ground, which might reveal a secret about human evolution.

These nests are essentially woven frames of broken branches made into sleeping platforms. That's reasonably elaborate for a creature that is seconds away from passing out, and it suggests the chimpanzees put a decent amount of thought and strategic planning into where they sleep each night. So, if they suddenly decide to sleep on the ground one night, it isn't likely to be some exhausted mistake.

That's why the chimpanzees of the Nimba Mountains in Guinea are so intriguing. These chimps aren't yet comfortable enough with humans for us to get really close, but researchers from Cambridge and the University of Kyoto have been able to use genetic analyses of hairs they leave behind in their nests to figure out which individual chimpanzees are sleeping where each night. It turns out these chimpanzees sleep on the ground or in the trees more or less interchangeably, and that it seemingly isn't that big a decision for them.

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That's surprising, because our general conception of our own evolutionary history tends to make leaving the trees to live on the ground a really big deal. The switch from arboreal to terrestrial living has been linked to the use of fire or massive environmental changes - basically, something that would spur a major evolutionary shift. But, if these chimps are anything to judge by, it really isn't that big a deal at all, and the ability to live equally comfortably on the ground or in the trees could go all the way back to the common ancestor of humans and chimpanzees. Head researcher Dr. Kathelijne Koops adds:

"This is intriguing as it has long been believed that coming down from the trees was a crucial evolutionary shift. However, this chimpanzees' behaviour suggests a more deep-seated, gradual transition from tree-to-ground sleep. These chimpanzees offer a rare opportunity to investigate why a population of wild apes chooses to sleep on the ground. We showed that ground-nesting was not caused by male mate-guarding behaviour, a lack of trees in which to nest, or because of fire. This suggests that our direct ancestors were neither the only, nor the first, species to come down from the trees."